Piezoelectric multilayer components, in particular multilayer piezo-actuators, are constructed as a sequence of piezoelectric ceramic layers and metallic internal electrodes, designated as piezo-stack hereinafter. In one conventional configuration, the internal electrodes extend as far as a surface of the component, such that at the outer side of the piezo-stack, owing to the high electric field strengths (typically 2 kV/mm), that usually occur, electrical flashovers can occur between internal electrodes that are at different electrical potentials. In order to avoid that, the piezo-stack is provided with a passivation which can form, in particular, an enclosure of the piezo-stack. The breakdown strength of the material of the passivation must be greater than the electric field strengths that occur, and this property must be maintained for all operating states of the component over the entire period of use. Moreover, the passivation should protect the component against external influences (for example against penetrating substances). In addition, the operation of the piezo-actuator places high demands on the mechanical properties of the passivation.
A passivation having the required properties can be produced from ceramic material which has a high breakdown strength and affords good protection of the piezo-stack against external influences. In order that the required properties are maintained, it is important for the passivation not to be damaged during operation of the component and not to acquire any cracks.
The piezo-stack is polarized during the production of the component in order to impress a remanent polarization on it. This results in an expansion of the piezo-stack. The piezo-stack is expanded again during operation of a piezo-actuator. Owing to the fixed connection between the passivation and the piezo-stack, the expansions of the piezo-stack are transmitted to the passivation, such that a tensile stress is built up therein. If the tensile stress exceeds a limit value, cracks form in the passivation.
If the same ceramic material as for the ceramic layers of the piezo-stack is provided as passivation, the passivation is likewise polarized during the polarization of the piezo-stack by the electric leakage fields occurring at the edges of the internal electrodes. That has the effect that during operation of the piezo-actuator the passivation is expanded together with the piezo-stack—although not as greatly as the piezo-stack—and the tensile stress built up in the passivation is lower than without expansion of the passivation. A prerequisite for this is that the passivation is only approximately at most as thick as an individual ceramic layer of the piezo-stack and can therefore be sufficiently polarized. The protection brought about by the passivation may be inadequate, however, owing to the small thickness.